Variable compression ratio (VCR) engines capable of varying the compression ratio of an air-fuel mixture gas to meet driving conditions of a vehicle are a known technology. The VCR engines can extract more torque by increasing the compression ratio under low load and can suppress knocking by decreasing the compression ratio under high load.
One technique of varying the compression ratio of the air-fuel mixture gas, that is, the ratio of a maximum to a minimum volume of a combustion chamber in a cylinder obtained by vertical movement of a piston is to change relative positions of a cylinder block and a crankcase by moving at least one of these components.
Here, the air-fuel mixture gas in the combustion chamber sometimes leaks out from a gap between the piston and the cylinder in the engine into the crankcase, etc. The leaked mixture gas is commonly called blowby gas and contains unburned fuel. The blowby gas is returned to an intake pipe through a crank chamber in the crankcase.
However, if the relative positions of the cylinder block and the crankcase are changed as mentioned above, the blowby gas, engine oil, etc. may flow out of the engine from a gap between the cylinder block and the crankcase and scatter and cause such problems as contamination of an engine-surrounding area and corrosion of metal parts around the engine.
As disclosed by Japanese Patent No. 5,313,284, that is, Japanese Unexamined Patent Publication No. 2012-202,371, it has been proposed to cover the gap between the cylinder block and the crankcase with an extendable tubular boot seal having a two-layer rubber structure. This boot seal comprises an outer layer formed of ethylene acrylic rubber and an inner layer formed of fluorine-containing rubber. The use of fluorine-containing rubber having good resistance to heat, oil and chemicals as an inner layer prevents the boot seal from degradation even when the boot seal is exposed to blowby gas.
However, the inner layer formed of fluorine-containing rubber is made thin for the sake of cost reduction. When a thin fluorine-containing rubber sheet is placed in a mold and ethylene acrylic rubber is injection molded on an outside of the sheet, a portion of the fluorine-containing rubber layer near an injection gate tears easily due to high injection pressure, and a portion of the fluorine-containing rubber layer where flows of the molten material join together creases easily. If the fluorine-containing rubber layer tears, the blowby gas will contact the outer layer rubber through a torn portion and impair durability of the outer layer rubber.
Moreover, Japanese Unexamined Patent Application Publication No. H11-188,757 discloses a method for forming an instrument panel having a two-layer structure by injection molding. A surface skin is placed in a cavity of a mold and then foamed resin is injection molded. In the technique of Patent Document 2 as well as the technique of Patent Document 1, injection pressure is applied on the surface skin and may tear or crease the surface skin.